The spinal column is a complex system of bones and connective tissue that protects critical elements of the nervous system. Despite these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion. Trauma, disease, or developmental irregularities can result in spinal pathologies that limit this range of motion.
Orthopaedic surgeons commonly perform procedures to correct spinal irregularities and restore stability to the spine through stabilization or immobilization of vertebrae within the spine. A stabilization procedure is often performed to alleviate the pain and discomfort that can be caused by a herniated or slipped disc. In such a procedure, the damaged disk is surgically removed from the space between adjacent vertebrae and an implant, such as a bone graft or cage, is then positioned between the adjacent vertebra. The implant promotes fusion of the adjacent vertebra thereby stabilizing the spine. A spinal stabilization system is sometimes used during fusion to stabilize the adjacent vertebra.
One example of a spinal stabilization system is disclosed in U.S. Pat. No. 6,050,997 to Mullane. This system includes spinal constructs having connective structures such as elongated rods that are secured adjacent the portion of the spinal column intended to be immobilized. Bone anchors, such as screws and hooks, are commonly utilized to attach the connective structures to the posterior surfaces of the spinal laminae. These components may provide the necessary stability both in tension and compression to achieve the desired stabilization of a portion of a spine.
Various fastening techniques and devices are used to secure bone anchors to the rod or cord of a spinal stabilization system. For example, in one system, the bone screws have a receiving slot or opening in a head portion of the screw for receiving a rod or cord. To accommodate connection to a rod or cord, many of these bone screws are open-ended at one end and have a yoke with a pair of upstanding arms that can receive the rod in a channel formed between the arms. The bone screws are implanted in predetermined vertebrae of the spine (adjacent vertebra, for example) and a rod or cord is then extended through the slot opening in each bone screw. The bones screws are then connected to the rod or cord by a set screw or nut that engages the rod or cord through or over a wall of the screw head. Tightening the set screw or nut causes the rod or cord to be forced or clamped within the head of the bone screw to provide a holding force that attaches the rod or cord to the bone screw. Applying a pre-specified torque to the screw or nut provides a rigid construct for indefinite duration.
With these mechanical fasteners, the contact area between the fastener and the spinal stabilization element is relatively small. For example, when a set screw is used to force a rod against an inside surface of a yoke of a bone anchor, the contact region between the set screw and the spinal stabilization element is generally limited to the diameter of the set screw. Since the spinal stabilization elements themselves are typically cylindrical, the contact with a flat surface of a set screw is limited to line contact. Shaping the screw contact surface to better correspond to the curved surface of a spinal stabilization element would enhance contact, but limits tightening of a threaded screw. Using a set nut provides only contact with a cylindrical rod or cord at two points. Because of this relatively small contact area, a substantial amount of torque in the range of 100 in. lbs. to 130 in. lbs. is needed in order to provide the required force needed to hold the rod or cord in place. Additionally, anti-torque tools are usually required to assist in tightening the fastener to the required torque. In many cases, one person is needed to torque the fastener while another person provides a countering force with an anti-torque tool.